Underwater acoustic communication involves sending and receiving messages below water via acoustic waves. Unlike a transmission medium such as air for terrestrial wireless communications using electromagnetic spectrum waves, the transmission medium for underwater messages is the water itself which presents a host of design challenges. Underwater communication is difficult due to factors such as multipath propagation, time variations of the communications channel, small available bandwidth, and strong signal attenuation, especially over long ranges. In most underwater communication systems, there are low data rates compared to terrestrial communication, since underwater communication typically employs acoustic waves instead of electromagnetic waves for transmission of data. Since most undersea communications is typically performed acoustically, communications are limited to a distance of about 200 meters at data rates of 10 kbps or less. The acoustic delay in water also severely restricts sound or ultrasound as a long range communication media, however.
Designing a reliable underwater acoustic channel presents a communication systems designer with many difficulties. Some distinguishing characteristics of the channel are frequency-dependent propagation loss, severe multipath distortions, and low speed of sound propagation. None of these characteristics are nearly as pronounced in land-based radio channels—the fact that makes underwater wireless communication extremely difficult, and necessitates dedicated system design. Some non-acoustic attempts have also been made to provide underwater wireless communications using waves other than acoustic waves. For example, the blue and blue-green regions of the electromagnetic spectrum have been used for undersea communications with limited results due to high signal attenuation in the 535 nanometer region of the spectrum, for example. Thus, with either acoustic and/or electromagnetic wave transmission in water, communication distance, data rate, and system reliability remain limited.